SummaryPartially purified extracts of soluble Hardy‐Passey mouse melanoma tyrosinase yielded three precipitin lines upon immunoelectrophoresis. Two immunochemically homologous but electrophoretically heterologous precipitin lines possessed tyrosinase activity whereas a more anodic line was observed which did not display tyrosinase activity.Tyrosinase from serum of patients with metastatic malignant melanomas cross‐reacted with antiserum to mouse melanoma tyrosinase, and one precipitin line was seen. Immunoelectrophoresis of mixed suspensions of human serum and mouse melanoma tyrosinase yielded two tyrosinase‐antibody complexes which intersected in a manner suggesting that the human and mouse melanoma tyrosinases possessed certain related antigenic g